1. Field of the Invention
The present invention relates to a semiconductor device including a semiconductor circuit comprising a semiconductor element, such as insulated gate type transistors, and a manufacturing method thereof. In particular, it relates to the technology for forming crystalline semiconductor films on top of insulating surfaces. The semiconductor device of the present invention is not limited to thin film transistors (TFTs), MOS transistor, and other insulated type gate transistor elements themselves, but instead include display devices, image sensors, and other electro-optical devices incorporating semiconductor circuits made from those insulated gate type transistors. In addition, the semiconductor device in the present invention includes electronic equipment incorporating the display devices or electro-optical devices.
2. Description of the Related Art
Active matrix liquid crystal displays, in which a pixel matrix circuit and a drive circuit are comprised of thin film transistors (TFTs) formed on top of a substrate with insulating properties, continue to be a center of attention. Liquid crystal displays from approximately 0.5 to 20 inches in size are in use.
Currently TFTs which have a crystalline semiconductor film as the active layer, such as the representative polysilicon film, are gathering attention in the drive to make a liquid crystal display that is capable of high definition display.
A crystalline semiconductor film is formed by initially forming an amorphous semiconductor film, such as the representative amorphous silicon film, and then crystallizing it. In general, chemical vapor deposition (CVD) is used to form the amorphous semiconductor film.
Until now, the low pressure CVD method, which can deposit a good quality amorphous silicon film, as well as the plasma CVD method, which has good throughput and deposits an amorphous silicon film at low temperature, have often been employed.
The speed that reduced pressure CVD can form an amorphous semiconductor film is slow, a disadvantage from a manufacturing perspective.
In addition, plasma CVD forms an amorphous semiconductor film by decomposition of a high priced reactive gas (monosilane, disilane, etc.) using sufficient RF power, but at that time, in addition to the amorphous semiconductor film, a large amount of a yellow powder is generated through a polymerization reaction. This powder is ultrafine, and is known to cause powder explosions when exposed to the atmosphere.
Therefore, due to the great danger of an explosion, a process that uses plasma CVD has a disadvantage from a workplace safety perspective.
Additionally, in the past, after the formation of the amorphous semiconductor film, several other processes are carried out (for example, crystallization, patterning) before the gate insulating film is actually formed. This means the surface of the crystallized silicon film, which is to become the active layer, is exposed to the atmosphere and may become either contaminated with impurities (oxygen, moisture, boron, sodium) or oxidized before the gate insulating film can be formed. Then when the gate insulating film is laminated on top, the active layer, especially the interface between the channel formation region and the gate insulating film, properties drop, and this causes a drop in the electrical characteristics of the TFT.
This is especially true for the atmosphere in clean rooms, in which boron (boric) from the HEPA type filter generally in use can cause boron to intermix with the exposed surface of the film in uneven concentrations. To make a glass mesh structure for the HEPA filter with ease, glass contains a high level of boron. In addition, after measuring the electrical characteristics of the TFT, it has been found that the boron inhibits crystallization during the semiconductor film crystallization process.